Coup de Grace

The finishing stroke. When criminals were tortured by the wheel
or otherwise, the executioner gave him a coup de grâce, or blow
on the head or breast, to put him out of his misery.

“The Turks dealt the coup de grâce to the Eastern empire.” —Times.

The following is taken from a note (chap. xxx.) of Sir W. Scott's
novel The Betrothed.

“This punishment [being broken on the wheel] consists in the
executioner, with a bar of iron, breaking the shoulder-bones, arms,
thigh-bones, and legs- taking alternate sides. The punishment is
concluded by a blow across the breast, called the coup de grâce,
or blow of mercy because it removes the sufferer from his agony.
Mandrin, the celebrated smuggler, while in the act of being thus
tortured, tells us that the sensibility of pain never continues after
the nervous system has been shattered by the first blow.”